Alex Zunger, National Renewable Energy Laboratory

Research Objectives

We are studying the electronic and optical structure of a range of semiconductor quantum dots at the atomistic level. The study focuses on both one-body electronic structure and many-body (configuration interaction) treatments.

Computational Approach

We have developed a parallel Folded Spectrum Method (FSM) code that allows us to find the exact near-edge eigenenergies and eigenfunctions of million-atom systems using an accurate, pseudopotential plane-wave description. We are using this code to study the electronic structure of million-atom quantum dots.

We have developed a set of codes to calculate the Coulomb, exchange, and polarization integrals associated with the near band edge levels calculated by the FSM. We are using this code to predict quantities such as singlet-triplet exchange splittings of excitons in quantum dots.

We have developed the first of a new generation of local-density approximation codes which feature many algorithmic advances over the standard parallel Car-Parinello or conjugate gradient codes currently in use.

Accomplishments

We have calculated the electronic structure of self-assembled InAs quantum dots embedded in GaAs. We have investigated the dependence of the optical properties on the different shapes, sizes, and composition profiles in the dots. Using these results to analyze measured optical spectra has enabled us to distinguish between several proposed models for the shape and composition profile of these quantum dots.

We have studied the difference between embedded (in GaAs) and freestanding InAs quantum dots. We found much larger quantum confinement effects in the freestanding quantum dots and big wavefunction spillage outside the quantum dot for the embedded quantum dot. We also found one unusual X-derived state bound outside the surface of the InAs quantum dot, induced by the compressive strain on the GaAs matrix. When pressure is applied to the system, this X state will cross through the internal Gamma derived states and quench the quantum dot photoluminescence. This finding has been confirmed by experiment.

The electronic structure of arrays of vertically stacked quantum dots has also been calculated. The dot-dot interactions have been analyzed in terms of the interaction between the strain profiles of the quantum dots and the coupling of their wavefunctions. This analysis has been used to interpret the optical spectra of quantum dot samples grown to contain multiple levels.

Isosurface plots of the charge densities of the valence-band states for the b=20a pyramids. The charge density equals the wave-function square, including the spin-up and -down components. The level values of the green and blue isosurfaces equal 0.25 and 0.75 of the maximum charge density, respectively.

Significance

The electronic, optical, transport, and structural properties of semiconductor nanostructures (films, quantum dots, and quantum wires) have potential applications in a whole new set of nanoscale devices such as lasers, sensors, and photovoltaics.

Publications

L. W. Wang, J. Kim, and A. Zunger, "Electronic structures of [110]-faceted self-assembled pyramidal InAs/GaAs quantum dots," Phys. Rev. B 59, 5678 (1999).

A. J. Williamson and A. Zunger, "InAs quantum dots: Predicted electronic structure of free-standing versus GaAs-embedded structures," Phys. Rev. B 59, 15819 (1999).

J. Kim, L. W. Wang, and A. Zunger, "Comparison of the electronic structure of InAs/GaAs pyramidal quantum dots with different facet orientations," Phys. Rev. B, Rapid Commun. 57, R9408 (1998).

http://www.sst.nrel.gov